Pergamon

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Adv Space Res. Vol. 24, No. 9, pp 1149-I 158, 1999 0 1999 COSPAR. Published by Elsevier Science Ltd. All rights reserved

Printed in Great Britain 0273-l 177/99 %20.00 + 0.00

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THE ROSETTA PLASMA CONSORTIUM: TECHNICAL REALIZATION AND SCIENTIFIC AIMS

J. G. Trotignon’, R. BostrGm*, J. L. Burch3, K.-H. Glassmeier4, R. Lundin’, 0. Norberg’, A. Balogh6,

K. Szegii7, G. Musmann4, A. Coates’, L. Ah&*, C. Carr6, A. Eriksson*, W. Gibson3, F. Kuhnke4,

K. Lundin’, J. L. Michau’, S. Szalai7

t Laboratoire de Physique et Chimie de l’Environnement, 3A avenue de la Recherche Scienttjique, F-45071

Orleans cedex 02, France

2 Swedish Institute of Space Physics, Uppsala Division, S-75591 Vppsala, Sweden

3 Southwest Research Institute, Instrumentation and Space Research Division, P. 0. Drawer 28510,

San Antonio, Texas 782280510, USA

4 Institutfir Geophysik und Meteorologie, Technische Universitat zu Braunschweig, Mendelssohnstrasse 3,

D-38106 Braunschweig, Germany

’ Swedish Institute for Space Physics, P. 0. Box 812, S-98128 Kiruna, Sweden

6 Imperial College, Space and Atmospheric Physics Group, The Blackett Laboratory, Prince Consort Road,

London SW7 2BZ, UK

7 KFKI Research Institute for Particle and Nuclear Physics, Department of Space Technology,

29/33 Konkoly Thege street, P. 0. Box 49, H-1525 Budapest, Hungary

’ Mullard Space Sci. Laboratory, University College London, Holmbuly St Mary, Dorking, Surrey RH5 6NT, UK.

ABSTRACT

The Rosetta spacecraft will rendez-vous with comet Wirtanen round 2011 and will study it for a period of nearly two years, thus providing a unique opportunity to monitor its behaviour over a wide range of distances from the Sun. A plasma and wave package, the Rosetta Plasma Consortium, RPC, will be part of the Rosetta orbiter payload. RPC is a highly integrated package that consists of five sensors: the Langmuir Probe, LAP, the Ion and Electron Sensor, IES, the Ion Composition Analyzer, ICA, the Fluxgate Magnetometer, MAG, and the Mutual Impedance Probe, MIP. RPC also includes the common instrument control, spacecraft interface, and power management unit, PIU, plus the Electrical Ground Support Equipment, EGSE. The prime objectives of RPC are to investigate: (1) the physical properties of the cometary nucleus and its surface, (2) the inner coma structure, dynamics, and aeronomy, (3) the development of cometary activity, and the microscopic and macroscopic structure of the solar-wind interaction region as well as the formation and development of the cometary tail. In addition, the planned asteroid, possibly Otawara and Siwa, flybys will provide an excellent opportunity to study in detail the physics of the solar wind-asteroid interaction. The magnetic and electric conductivity properties of the asteroid will also be studied. 0 1999 COSPAR. Published by Elsevier Science Ltd.

INTRODUCTION

The use of space probes allowed plasma scientists to considerably increase their knowledge of cometary nuclei and of the interaction between comae and the interplanetary medium. In 1985, the ICE spacecraft met for the first time a comet, Giacobini-Zinner, and traversed its tail at 7,862 km from the nucleus. ICE carried instruments for making measurements of plasmas, energetic particles, waves, and fields (Brandt et al., 1985). Six months later, in

1149

1150 J. G. Trotignon et al.

early March 1986, a fleet of five spacecraft, Vega 1, Suisei, Vega 2, Sakigake and Giotto successively visited the very active comet Halley, at distances lying between 596 km and 7 x106 km, on the sunward side of its nucleus. The two identical Soviet spacecraft, Vega 1 and 2 carried a scientific payload including plasma, gas and dust detectors, plus electric and magnetic field sensors (Sagdeev et al., 1986). The two spacecraft Suisei (Comet in japanese) and Sakigake (Pioneer) carried, respectively, a spherical electrostatic energy analyzer, and four sensors, a Faraday cup to collect solar wind ions, a magnetometer to measure the interplanetary magnetic fields, and a dipole antenna, 10 m long, combined with a search coil to investigate electromagnetic plasma waves (Hirao and Itoh, 1985). The Giotto payload included ion and electron spectrometers, a magnetometer and plasma experiments to investigate magnetic fields and plasma characteristics (Reinhard and Dale, 1980). Finally comet Grigg- Skjellerup was encountered by Giotto during its extended mission in july 1992, when it passed within 200 km of the nucleus on the dark side, thus making its second flyby of a comet (Schwehm, 1992).

Nineteen years after the Grigg-Skjellerup encounter by Giotto, the Rosetta spacecraft and its lander will study comet 46PWirtanen. The prime scientific objective of the Rosetta mission is to study the origin of comets, to establish the relationship between cometary and interstellar material, and to determine their implications in the solar system origin (Verdant and Schwehm, 1998). The Rosetta mission will provide a unique opportunity to investigate a comet and its environment during almost two years, from its asteroid-like behaviour, at a heliocentric distance greater than 3 AU, until its magnificence near perihelion, when the gas and dust production rates reach their maximum. A complex strategy will allow a rendez-vous with comet Wirtanen, remote observations, comet characterization, a lander deployment for in-situ measurements, and an extensive study of the cometary activity

(Pellon Bail&t et al. , 1998).

Comet 46PIWirtanen is a short-period comet (5.46 years), its nucleus is very small, 1.2-1.4 km by assuming an albedo of the nucleus surface of 0.04 (Mohlmann, 1996). Given its smallness the nucleus can be rated as faibly active, producing about 1.6 x lo’* water molecules per second at perihelion. The nucleus is believed to rotate very rapidly in 10 h-l day (retrograde rotation in 6 h or multiples of 6 h according to Miihlmann, 1996). The temperature of the nucleus surface should be about 210 Kelvins at 1 AU from the Sun, let us note that the temperature of a black body is 390 Kelvins. Finally the dust to gas mass ratio is thought to be between 1 and 1.2.

Among the spacecraft that have already encountered a comet, all of them carried plasma experiments, but as said before, only three, ICE, Vega 1 and 2 performed plasma wave measurements (Grard et al., 1987; Trotignon et al., 1991). Luckily, a plasma and wave package: the Rosetta plasma consortium, RPC, will be part of the Rosetta orbiter payload. RPC and the onboard radio science experiment will study the comet plasma and wave environment and its interaction with the interplanetary medium. In this paper some elements of cometary plasma physics are recalled, then the RPC plasma package is presented.

COMETARY PLASMA PHYSICS

Since the pioneering papers of Biermann (1951) and Alfven (1957) many theoretical studies or ground- and space-based observations have been performed to try to understand the interaction between the solar wind and comets. The large scale features of this interaction near perihelion are illustrated in Figure 1. The Giotto mission has confirmed that the cometary nucleus can be considered as a ball of ice and dust which consits of a mixture of frozen volatiles and non-volatile components. Most of the surface of the nucleus is covered with an inert crust, that means that the volatile elements evaporate from only a few small areas on the surface when the comet is close enough to the Sun (about 3-4 AU). Roughly

Fig. 1. Large scale features of the comet-solar wind interaction near perihelion (not to scale).

The Rosetta Plasma Consortium, RPC 1151

85% by weight are water vapor and most of the remaining 15% are molecules of carbon dioxid. The volatiles evaporate (sublimate) because the nucleus surface is warmer than the sublimate temperature (up to 300-400 Kelvins compared with, for example, 190 Kelvins for the water sublimation temperature). Since the escape velocity from a comet is generally 1 m/s or less, the released material quickly generates a rapidly expanding and gravitationally unbound cloud of gas and dust, the coma. At distances down to a few thousands kilometers from the nucleus, cometary gases have been observed to flow radially outward with a speed of about 900 m/s. Such a coma streches out as far as lo5 km from the nucleus and even more (the hydrogen coma composed of neutral hydrogen atoms has been seen at several millions of kilometers from the nucleus). Throughout the coma, molecules are broken in smaller ones or atoms which may be ionized in turn. The ions of cometary origin that are mainly created by photoionization, electron impact ionization, or charge exchange are captured by the solar wind plasma and its embedded magnetic field. This process called ion pickup is thought to be responsible for the solar wind-comet interaction. Because ions are continuously produced, the solar wind carries more material along with it, thus it is mass-loaded. Conservation of momentum and energy implies a slowing down of the solar wind flow. The process continues until the inward pressure exerted by the solar wind and its magnetic field is balanced by the outward pressure exerted by cometary gases. This boundary, called contact surface, is the outer edge of a diamagnetic cavity, which contains no magnetic field of solar wind origin since the solar wind cannot penetrate beyond this surface. The width of this cavity was 8,500 km along the Giotto’s trajectory (Neubauer et al., 1986). Neither of the other spacecraft, even Giotto itself when visiting comet Grigg-Skjellerup, entered the diamagnetic cavity. As the number of ions picked up in the solar wind decreases substantially on the flanks of the comet, simply because fewer ions are created in these regions, the solar wind moves faster here than on the front side. Thus the magnetic field lines frozen into the solar wind plasma are caught up in the front of the coma and their ends are pulled downstream to form a magnetic tail. This phenomenon is known as the magnetic field draping effect (Alfven, 1957). The existence of a weak shock ahead of comets was predicted by Biermann et al. (1967). Such a shock forms at a critical point in the mass-loaded flow, when the mean molecular weight of the plasma reaches a critical value. This point strongly depends upon the gas production rate of the cometary nucleus (see for example Coates et al., 1997, and references therein). The shock was estimated at 400,000 km from the Halley nucleus along the comet-sun line from the Giotto observations. This position was confirmed by the Suisei and the two Vega spacecraft. The Giacobini-Zinner shock crossed by ICE is almost one order of magnitude closer to the nucleus, thus reflecting the lower gas production rate of this comet, 4 x 10” molecules per second (Mendis et al., 1986) when compared with Halley, 6.9 x 10z9 molecules per second (Krankowsky et al., 1986). Grigg-Skjellerup, which is a weakly active comet, with a neutral gas production rate of 6 x 10” molecules per seconde (Johnstone et al., 1993) exhibited a shock at 10,000 km in the sunward direction during the Giotto encounter in 1992. Within the shock, the solar wind flow decelerated and subsonic velocities occurred.

These well established large-scale features of the interaction between comets and the solar wind were predicted before the Halley, Giacobini-Zinner and Grigg-Skjellerup encounters by spacecraft. They are also satisfactorily modeled by MHD simulations. Recently Gombosi et al. (1996) developed a three dimensional multiscale MHD model that takes into account most of the interaction processes that are known, or believed, to play a significant role in cometary plasma physics.

0. -10

X (1000 km)

Fig. 2. (adapted from Gombosi et aE., 1996) Inner shock (thin line), plasma cavity boundary layer (thick line), plasma stream lines (dotted lines), magnetic field lines (dashed lines), and pile-up region (shaded area) in the equatorial plane. These structures of an active Halley class comet are predicted by a 3-D multiscale MHD model.

1152 J. G. Trotignon et al.

By including these processes (photoionization, electron impact ionization, charge exchange, electron-ion recombination, as well as ion-neutral friction effect) and using a multiscale approach the authors are able to address the difficult problem of describing both the outer regions (such as the upstream solar wind and the shocked solar wind) and the inner coma (with the diamagnetic cavity and its internal shock). To overcome one of the limitations inherent to single-fluid MI-ID models, namely the impossibility to treat the electron density and temperature in a self-consistent manner, observed and/or empirical radial profiles have been used.

The Rosetta spacecraft will stay most of the time within a few hundred km of the Wirtanen nucleus. It will eventually make a few excursions into the inner coma and the plasma tail at distances less than 10,000 km from the

nucleus during the Extended Monitoring Phase (Pellon Bailon et al., 1998), but mainly the spacecraft will stay

inside the contact surface’near perihelion. For this reason we will now focus on the inner coma processes and in particular those arising in the contact surface and around it. The physical processes that are believed to play a major role in the innermost coma region are ionization, recombination, and ion-neutral friction (see Gombosi et al., 1996, and references therein). Recombination occurs mainly between electrons and HzO’, H30’ ions, which are the majority within 20,000 km from the nucleus, and is primarily controlled by the electron temperature. Inside the contact surface all electrons are cooled by collisions with ionized cometary molecules. Both the electrons and ions of the water family are strongly coupled to the radially expanding neutrals by ion-neutral friction. The plasma particles and neutrals are travelling in convoy at about 1 km/s. As this velocity turns out to be greater than the

m Halley Ill Bow shock

formation

II Non-linear structurina

h-structured Yfiycloidal motion

10-l I I I 1 I I I I 0 5 10 15 20

Mach number, MA

Fig. 3. (from Sauer et al., 1996) Predicted bi-ion fluid regimes that should be encountered by the Rosetta spacecraft during its study of comet Wirtanen. Sk = 10.” Q, where Q is the neutral gas production rate.

sound velocity, an inner shock is formed. Gombosi et al. (1996) have shown that many details actually observed by Giotto around the contact surface could be reproduced by their refined MHD model. The contact surface and inner shock, which is very close to it on the sunward side (a few hundred km), exhibit a teardrop shape (Figure 2 adapted from Gombosi et al., 1996). The inner shock is 2,200 km and 15,500 km from the nucleus, respectively, on the sunward and dark sides, while the terminator width is 6,400 km. The magnetic field lines pile up in front of the diamagnetic cavity (magnetic barrier) and then spread outward in both directions to form the plasma tail. Another region of increased magnetic field is also observed, 10,000 km ahead of the nucleus, An ion density peak was detected in this region known as the pile-up region (Balsiger et al. , 1986). This density peak and the ion density depletion region (Neugebauer, 1990) have been linked to a local change of the electron temperature that is believed to be a permanent feature of active comets (Gombosi et al., 1996; Sharma and Milikh, 1996). As a consequence of the rapid increase of the electron temperature, the recombination rate sharply decreases and becomes negligible beyond 5 x 104 km from the nucleus. Thus ionization locally exceeds recombination and an ion peak occurs. The ion density peak is responsible for an increase of the local plasma pressure and therefore an increase of the magnetic field.

The Rosetta Plasma Consortium, RF’C 1153

The Rosetta mission will rendez-vous with comet Wirtanen at a large distance from the Sun, beyond 3 AU, and will subsequently follow it down to its perihelion, about 1 AU. In other words, the neutral gas production rate will vary over a wide range, from 6 x 102’ at 3.5 AU to 1.6 x 102” at 1 AU (Enzian and Schwehm, 1997). It is worth noting that MI-ID models will no longer be valid when the extended comet interaction region becomes of the order of one heavy ion gyroradius or less in the free solar wind, i.e. for low gas production rates. Such an interaction has never been observed in-situ if we except the artificial comet-like interaction provided by the AMPTE Ba and Li releases. Two-dimensional collisionless bi-ion fluid simulations performed by Sauer et al. (1996) and Bogdanov et al. (1996) have shown that for low nucleus activities bi-ion discontinuities may periodically be produced in regions where the cometary plasma density is of the same order as the solar wind density. These discontinuities arise when the heavy ion motion comes into resonance with a bi-ion plasma mode of very low phase velocity. Three regimes (the non-structured cycloidal motion, the non-linear structuring, and the bow shock formation) have been identified as a function of the neutral gas production rate, Q, and the solar wind Mach number, MA. Figure 3 (from Sauer et aZ., 1996) predicts the regimes that should be encountered by the Rosetta Spacecraft all along its trajectory. For comparison, the Q and MA parameters encountered during the Halley, Grigg-Skjellerup and Giacobini-Zinner explorations are also indicated (Sh,, = 10.” Q). Finally, let us note that a cavity with a radius of 10 km requires a production rate of 2 x 10” molecules per second, which turns out to be a minimum value for the existence of a diamagnetic cavity (Sauer et al., 1996).

THE ROSETTA ORBITER PLASMA CONSORTIUM

RPC Overview

The Rosetta orbiter plasma consortium, RPC, will address several of the prime scientific objectives of the Rosetta mission, namely: (1) the physical properties of the cometary nucleus and its surface, (2) the inner coma structure, dynamics, and aeronomy, (3) the development of cometary activity, and the microscopic and macroscopic structures of the solar-wind interaction region as well as the formation and development of the cometary tail.

RPC is composed of five sensors, a Langmuir Probe, LAP, an Ion and Electron Sensor, IES, an Ion Composition Analyzer, ICA, a Fluxgate Magnetometer, MAG, and a Mutual Impedance Probe, MIP. All sensors of the RPC package are served onboard by the Plasma Interface Unit, PIU, a common instrument control, spacecraft interface, and power management unit, and on ground by a common Electrical Ground Support Equipment, EGSE. Measurements of electron density and temperature will be provided by LAP (continuous monitoring of variations) and MlP (absolute measurements and access to very low temperatures, possibly 30 K). Both MIP and LAP will provide important measurements of the velocity of the plasma that flows outward from the comet nucleus and also monitor the natural waves at low frequencies with LAP (O-10 kHz) and higher frequencies with MIP (10 kHz-3.5 MHz). The ion and electron distribution functions will be determined by the IES tophat electrostatic analyzer (over the 3 eV/e - 30 keV/e energy range) while ICA will mainly investigate the ion composition (over the 1 - 1012 amu mass range). In addition, magnetic field measurement will be performed with a high sensitivity by MAG. The accommodation of the sensor and the RPCO box that includes the PIU and the LAP, MAG, and MIP electronics boards is shown in Figure 4. The current RPC experiment mass and power consumption figures are indicated in

Table 1. RPC Mass Breakdown (Kg)

MIP Sensor 0.370 MAG Sensors 0.096

PIU + LAP, MAG

and MIP Electronics 3.291

Harness 1.064

Total 8.284

Table 2. RPC Power Breakdown

(Nominal Secondary Power, W)

I IES 1 1.850 I

ICA 4.241

LAP 1.380

1154 J. G. Trotignon et al.

Tables 1 and 2, respectively. The responsible group, the Principal Investigator or Investigator and the Technical Manager of each element of the Rosetta orbiter plasma consortium are given in Table 3.

Table 3. RPC Instrument Leaders and Institutes

Instrument Responsible Groups PI or I Technical

Manager I

LAP

IES

ICA

IRF-U, Uppsala R. Bostrijm (PI) L. Ahlen

SWRI, San Antonio J. L. Burch (PI) W. Gibson

IRF-K. Kiruna R. Lundin (PI) K. Lundin

MAG

MIP

PIU

TU-Braunschweig

LPCE, OrlCans

ICSTM, London

K.-H. Glassmeier (PI) F. Kuhnke

J. G. Trotignon (PI) J. L. Michau

A. Balogh (I) C. Carr

EGSE KFKI-RMKI,

Budauest K. Szegii (I) S. Szalai

I

Once a year a RPC scientific chairman (spokesman) is chosen among the five PIs, the current spokesman is J. G. Trot&non. G. Musmann holds permanently the RPC technical manager post. Finally, the RPC science team is under the chairmanship of A. Coates from MSSL. Two of the co-authors of this paper are not listed above, 0. Norberg from IRF-K is the Co-PI of ICA while A. Eriksson from lRF-U is the LAP project manager.

RPC Detailed Descrintion

Lanmnuir Probe. LAP. The LAP experiment is composed of two classic Langmuir probes for the measurements of the plasma density and temperature, the density variations and waves, the plasma flow speed, the spacecraft potential, and the low frequency electric fields. The probes are spherical, 5 cm in diameter, and made of titanium to stand the harsh environment of dust impacts and the presence of reactive gases. They are mounted on 20 cm short booms which are themselves fixed at the tips of the two long boom provided by the Rosetta project (see Figures 4 and 5). LAP will mainly address the following topics: the structure and dynamics of the comet Wirtanen outgassing, the relation of the coma structures to the comet surface properties, the plasma structures and boundary layers, the waves and fields in the coma, and possibly the tail processes.

Ion and Electron Sensor, IES. The IES is an electrostatic analyzer, featuring electrostatic angular deflection to obtain a field of view of 90” x 360”. The instrument objective is to measure ion and electron distribution functions over the energy range extending from 3eV/e up to 30 keV/e with 4% energy resolution. The angular resolution for electrons is 5” x 22.5” (18 azimuthal x 16 polar sectors), 5” x 45” (18 azimuthal x 8 polar sectors) for ions, and 5” x 5” (1 polar angle sector) for solar wind ions. A 3D distribution function is delivered each 65.5 s. The charged particle optics of IES are based on a toroidal tophat geometry that gives maximum aperture area for a given analyzer size. The electron analyzer is mounted on top of the ion analyzer, and the two analyzers, which have identical geometry except that the ion analyzer is scaled up for a higher geometric factor, share common digital electronics (Figure 5).

Ion Comnosition Analvzer. ICA. ICA uses the same toroidal electrostatic analyzer approach as IES with the addition of a time-of-flight velocity analyzer for mass/charge analysis. The ICA detector consists of an electrostatic entrance angle selection system, an electrostatic tophat analyzer with 360 degree field of view, followed by a cylindrical momentum analyzer based on permanent magnets. The particle detection system provides simultaneously the azimuth and mass per unit charge (m/q) of the incident ion distribution in a 16 x 32 (angle x mass) matrix. The mass detection technique utilized by ICA is ideal for the detection of ions with a wide range of masses (1 to approximately lo’* amu), including submicron sized dusty plasma components. ICA operates over the energy/charge range from leV/q to 40 eV/q with 7% resolution. A 2D distribution is delivered each 4 s and a 3 D distribution each 64 s.

The Rosetta Plasma Consortium, RPC 1155

Comet

l IES

I I-.

\ .

t . A P l

qy

Comet

LZ

MIP IES

RPCO

MAG1

L A P 2

Fig. 4. Schematic views of the Rosetta spacecraft with the RPC sensors and the RPC0 box that includes the PIU and the electronics of LAP, MAG, and MIP. The booms are shown in their stowed and deployed positions at the top and bottom, respectively.

Fluxgate Magnetometer, MAG. The MAG fluxgate magnetometer aims to measure any magnetic properties of the cometary nucleus, either permanent or induced, as well as the magnetic field in the comet-solar wind interaction region during the changing activity of the comet. To measure the magnetic field a system of two ultra light triaxial

1156 J. G. Trotignon et al.

Ion Electron Sensor,. IES

Ion Composition Analyzer, ICA Fluxgate Magnetometer, MAG

Plasma Interface Unit, PIU

Mutual Impedance Probe, MIP

Electrical Ground Support Equipment, EGSE

5. The Rosetta Plasma Consortium is composed of five sensors, LAP, IES, ICA, MAG, and ‘MIP, and two common facilities, the onboard PIU and ground EGSE.

Fig

The Rosetta Plasma Consortium, RPC 1157

fluxgate magnetometers are built, with the outboard sensor mounted close to the tip of the long spacecraft boom pointing away from the nucleus and with the inboard boom about 10 cm to 30 cm closer to the spacecraft body (Figure 4). Two magnetometer sensors are required to correct for the influence of the rather complex spacecraft field on actual measurements, and for redundancy purposes. In order to achieve the scientific requirements the spacecraft magnetic DC-field at the outer magnetometer sensor should not exceed 25 nT. To achieve this goal a magnetic cleanliness programme has been setup, conducted by the experimenter team, supported by the Rosetta project. MAG measures the magnetic field in the +/- 16384 nT range by step of +/- 0.031 nT. The temporal resolution is 1 vector/s in the nominal mode and 50 vectors/s in the maximum mode.

Mutual Impedance Probe. MIP. The MIP measures the electrical coupling of a transmitting antenna and a receiving antenna, and identifies the plasma density, temperature and drift velocity from the features of the frequency response. The mutual impedance of the antenna is only influenced only by the properties of the surrounding plasma. Its performance is consequently not affected by the shape, cleanliness, finish and photoemissive properties of the sensor, nor by the lack of uniformity of its surface potential. Extremely low energetic plasma can then be explored, an important advantage in a medium where temperatures as low as 100 K have been predicted. In the passive mode, MIP has the capability of a plasma wave analyzer. It detects the electric fields of waves associated with the interaction of the solar wind with the charged dust and ionized outgassing products of the nucleus, as well as the impulsive signals generated by individual dust particles impacting the spacecraft surface. These observations will therefore provide a continuous monitoring of the nucleus activity.

Plasma Interface Unit. PIU. The PIU interface and control unit lies between the spacecraft and the five RPC sensors. It provides a single interface to the spacecraft. The PIU incorporates the power conditioning and switching unit which converts the spacecraft supplied voltage to the levels required by the sensors. Dual redundant DC/DC converters provide the five different voltages required by the sensor units. Voltages are closely regulated according to the stringent requirements of the sensors. Command packets from the spacecraft are decoded and executed on the respective sensor. The science and housekeeping data are collated and packetised before transmission to the spacecraft. The digital electronics is based on redundant dual processor technology developed at Imperial College, providing a high degree of reliability and fault-tolerance. In addition to the hardware contribution, Imperial College will also take a lead role in the integration and testing of the RPC.

Electrical Ground Sunoort Eauinment, EGSE. The task of the EGSE is to support the RPC Package instruments check-out as a whole system at unit level and system level tests, as well as to support flight operation. It can be used to decommutate the RPC data from the telemetry stream and can serve as a quick-look facility. As the RPC Package calibration will be done at sensor level, the EGSE does not have to support calibration. The EGSE consists of a commercially available computer configuration (IBM-PC) upgraded with hardware units to simulate the spacecraft communication interface, a power supply, and interfaces for LAP, IES and ICA sensor stimulators (Figure 5). The EGSE will process and analyse housekeeping and science data both in real time and from archives, to uplink data loads to the instrument, and to accept real time auxiliary information Checkout System. The configuration has adequate storage capability for temporary data data storage.

from the Central Rosetta and it supports permanent

CONCLUSION

The orbiter Rosetta plasma package, RPC, is a fully integrated, light (8.3 kg) and coherent set of plasma and wave instruments to investigate the comet Wirtanen nucleus and coma during all its activity phases. The measurements will be performed as close to the nucleus as one kilometer, at about 4 AU from the Sun, up to several hundreds kilometers (maybe thousands if some excursions in the plasma tail are possible) near perihelion. In this way the Rosetta spacecraft will spend most of its time in the innermost coma regions. In particular it will investigate chemical and physical processes inside the contact surface when it will exist (from roughly 1.6 AU to perihelion). Unlike the previous comet flybys the Rosetta spacecraft to comet Wirtanen relative velocity will be very low (less than one meter per second instead of several tens of kilometers per second). On its way to Wirtanen, Rosetta will inspect two asteroids, possibly Otawara (on 11 July 2006) and Siwa (on 24 July 2008). Their interactions with the interplanetary medium will then be studied with the RPC instruments. Finally, Rosetta will perform one Mars’ (on 26 August 2005) and two Earth’s (on 28 November 2005 and 28 November 2007) gravity assists to gain enough

1158 J. G. Trot&on et al

orbital energy to reach Wirtanen. These planet fly-by should be of great benefit to the RPC experimenters to check their instruments and also collect valuable data in planetary environments, especially in the ionosphere of Mars.

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